Parallel resonant push-pull inverters, series resonant push-pull and half-bridge inverters, and these and other variations of non-resonant inverters have all been used in an attempt to power discharge lighting. Though some are successful in accomplishing the objective, there are drawbacks which would indicate that an alternative method would be more suitable. Parallel resonant type inverters impose extremely high voltage demands on the switching transistors thus increasing cost and offer no inherent load imbalance correction. Series resonant type inverters usually use a high voltage generated across a capacitor through series resonance to cause ignition of the lamp and then revert to non-resonant operation as the series capacitor is shunted by the lamp after ignition. Therefore, these types of inverters as well as the non-resonant types suffer from high levels of RFI generation through square wave operation, an operating frequency which is usually dependant on input voltage and/or load current, as well as reduced switching efficiency as compared to inverters which operate continuously in the resonant mode.
The prior art also encompasses a half-bridge resonant inverter which is composed of two matched resonant circuits on the primary side of the transformer. This circuit, however, is of complex construction and difficult to reliably produce as both primary resonant circuits must be exactly matched.
It is well known by those versed in the art that high intensity discharge lamps require an initial very high voltage pulse to start the arc and that the value of the voltage required is dependant upon the duration of the pulse which cannot exceed the time duration of one half of the oscillating frequency. As the frequency increases, the value of the starting voltage increases and may be many thousands of volts. These starting voltages are both dangerous and expensive to contain. As a consequence, current electronic inverter ballasts for these types of lamps are both expensive and complex to manufacture.
Examples of prior art circuits are shown in the following patents:
______________________________________ 4,513,226 4/1985 Josephson 4,469,988 9/1984 Cronin 4,277,728 7/1981 Stevens 4,277,726 7/1981 Burke 4,259,616 3/1981 Smith 4,259,614 3/1981 Kohler 4,245,177 1/1981 Schmitz 4,237,403 12/1980 Davis 4,199,710 4/1980 Knoll 4,127,795 12/1978 Knoll 4,060,752 11/1977 Walker 4,060,751 11/1977 Anderson 4,004,187 1/1977 Walker 3,754,160 8/1973 Jensen ______________________________________